Method of carburizing steel and steel alloys, and fused salt bath thereof



METHOD @F CARBURHZING STEEL AND STEEL ALLOYS, AND FUSED SALT BATH THEREOF Frederick David Waterfall and Robert Leslie Hewson,

Oldhuiy, England, assignors to imperial Chemical ludnstries Limited, London, England, a corporation of Great Britain No Drawing. Application March 23, 1956 Serial No. 573,339

Claims priority, application Great Britain March 25, 1955 '7 Claims. (Cl. 148-455) This invention relates to improvements in the heat treatment of steels and alloy steels and to improved fused salt baths and compositions therefor.

It is known that fused salt baths of widely difierent composition may be employed for the heat treatment of steels and that steel articles immersed in such salt baths will emerge after treatment carburised by the action of the mixture of fused salts, the depth of carbon penetratration depending on the precise conditions under which treatment is carried out. Thus a good depth of carbrised layer and good degree of saturation in the layer may be obtained when treating steel articles in a fused salt bath consisting of one or more of the chlorides of the alkaline earth metals barium and strontium, alkali metal cyanide and a small proportion of alkali metal chloride. In most cases where such molten baths are used for heat treatment purposes it is usual to cover the molten salt mixture with a layer of graphitic carbon so as to prevent fuming and undue oxidation of the melt. These aforesaid baths, however, sulfer from the defect that they leave on the steel articles treated a deposit which cannot readily be removed by washing with water; the articles therefore require to be specially cleaned after treatment in the bath and if salt deposit is not entirely removed rusting is liable to occur.

According to the present invention an improved fused salt bath for the heat treatment of steels and alloy steels comprises alkaline earth metal halide, alkali metal halide, to 30% of alkali metal cyanide, and 0.25% to 5% by weight of a regenerator, namely, titanium or titanium dioxide or any combination of one or both of these with silicon or silica, the percentages indicated being calculated on the total weight of the bath.

The present invention includes within its scope com positions adapted to produce fused salt baths of the type described and also includes methods for the heat treatment of steels or alloy steels which comprise heating the steel or alloy steels in a bath obtained by fusing such compositions.

Titanium is preferably employed as regenerator. We have found that although titanium dioxide may be employed as an alternative to titanium less useful results are obtained. For instance in contradistinction to titanium when using titanium dioxide as regenerator intergranular penetration of the surface layers of alloy steels is liable to occur.

Suitably the regenerator is present in the bath in a form having a large ratio of surface to volume, such as powder. Addition of the regenerator to the bath is conveniently effected by enclosing powdered regenerator in a bag and adding to the bath or by mixing the powdered regenerator with a small amount of one or more 2,845,344 Patented Aug. 5, 1958 ice of the salt constituents of the batch, for instance, a salt mixture of the same composition as the original fused melt and adding the mixture of regenerator and salt to the bath.

Although useful results may be obtained when operating the bath containing up to 5% of the regenerator a considerably lesser quantity than the said 5% may be employed. The particular proportion of regenerator to be added depends on-the conditions under which the bath is operated including the amount of steel or alloy steelarticles treated in the bath. Generally, however, when using the preferred regenerator, that is titanium in the form having a large ratio of surface to volume such as powder good results have been obtained when said titanium is present in the bath in the proportion of 0.5% to 2%, suitably 0.75% by weight of the bath. Periodic addition may be made to the bath of these latter mentioned proportions of the preferred regenerator, suitably per 8 hour day, depending somewhat on the conditions under which the bath is employed.

The bath may be operated at any temperature above the fusion point of the salt mixture and as the temperature is increased above this point the rate of carburisation became greater. The actual temperature employed is dependent on the composition of the salt bath but generally we prefer to operate at a temperature of 800 C. to 950 C.

One suitable method of operating the bath is first to make a melt of an alkaline earth metal chloride and an alkali metal chloride and maintain the melt at a temperature of 800 C. to 950 C. 5% to 20% of sodium cyanide is added to the bath and approximately 0.75% of titanium powder is dispersed in the bath, by either of the methods previously described. Melts containing various amounts of the salt components may of course be made up and a suitable one contains 15% to 25% of barium chloride and 15% to 25% of sodium cyanide, the remainder being alkali metal chloride and regenerator. The bath is dredged at regular intervals, preferably each day and the sludge formed in the process of carburising the steels is removed. At such intervals the bath is made up to the working level by further additions of alkaline earth and alkali metal chloride. Suflicient sodium cyanide is also added to make good losses. When operating the bath for an 8-hour day a daily addition of approximately 0.75% by weight of powdered titanium may suitably be added. When making up the bath after the addition of titanium it may also be a convenient time to add powdered graphite to the bath so as to form a layer on the surface and it must be understood that the present invention envisages the presence of a powdered graphite layer on the bath Whenever desirable.

The baths of the present invention are superior to those made up from alkaline earth metal halide, alkali metal halide and alkali metal cyanide and containing no regenerator. For instance, the water solubility of our baths is greater, so that salts adhering to the steel articles treated in our baths are more readily removed when washed with water than is the case with said other baths.

The following examples illustrate the invention. Where percentages are mentioned they are by weight, calculated on the total weight of the bath.

Example 1 A fused salt bath of weight lbs. containing 55% BaCl 12% NaCN, 16.5% KCl, 16.5% NaCl was made up and heated to 950 C. 0.75% of titanium powder contained in a linen bag was then added to the bath and the surface of the bath was covered with a layer of graphitic carbon and pieces of carbon casehardening steel as described in British Standard Specification 97011947 No. En 32 were immersed in the bath for a period of 2 hours. The bath was in use for 80 days, the sludge being removed daily and a mixture containing 55% BaCI 20% NaCN, 12.5% KCl and 12.5% NaCl was added daily to replenish salt losses 0.75% of titanium powder was also added daily to the bath in the manner described.

The analysis of the carbon casehardening steel described in British Standard Specification 970:1947 No. En. 32 is as follows:

Percent Carbon 0.10-0.18 Silicon 0.05-0.35 Manganese 0.61.0 Sulphur 7 0.07 max. Phosphorous 0.05 max.

The bath was analysed from time to time and after treatment the steel articles were allowed to cool slowly and examined. The following results were obtained:

Sodium Total Case Eutectoid Cyanide Case Depth, Depth, Content Depth, 0.6% 0.9% of Bath, inches Carbon, Carbon, percent inches inches Test pieces of an alloy steel containing nickel and chromium as described in British Standard Specification No. S82 treated in this bath for 3 hours at 900 C. were free from intergranular penetration.

The analysis of the alloy steel containing nickel and chromium as described in British Standard Specification No. 8.82 is as follows:

Percent Carbon 0.12-0.18 Silicon 0.10-0.35 Manganese 0.5 max. Nickel 3.8-4.5 Chromium l.0-1.4

Example 2 Sodium Total Case Eutectoid Cyanide Case Depth, Depth, Content Depth, 0.6% 0.9% of Bath, inches Carbon, Carbon, percent inches inches On the 5th and 14th day test pieces of an alloy steel fit described in British Standard Specification No. 8.82 were treated in the bath for 3 hours at 900 C. and no intergranular penetration of the surface layers was observed.

Example 3 A fused salt bath was made up, run and replenished as in Example 1 but in this case 0.64% of powdered titanium and 0.11% powdered silicon were added daily to the batch (in the manner described in Example 1).

The bath was analysed from time to time and after treatment for 2 hours at 950 C. carbon casehardening steel pieces as described in Example 1 were allowed to cool slowly and examined. The following results were obtained:

Sodium Total Case Eutectoid Cyanide Case Dept-h, Depth,

Content Depth, 0.6% 0.9%

of Bath, inches Carbon, Carbon,

percent mches inches On the 12th day test pieces of an alloy steel described in British Standard Specification No. 8.82 were treated in the bath for 3 hours at 900 C. and no intergranular penetration of the surface layer was obtained.

Example 4 A fused salt bath was made up, run and replenished as in Example 1 but in this case 1% of titanium dioxide was added daily to the bath.

The bath was analysed from time to time and after treatment for 2 hours at 950 C. carbon casehardcning steel pieces as described in Example 1 were allowed to cool slowly and examined. The following results were obtained:

Sodium Total Case Euteetold Cyanide Case Depth. Depth, Content Depth, 0.6% 0.0%

of Bath, inches Carbon, Carbon, percent inches 1 inches.

On the 8th, 16th and 21st days test pieces of an alloy steel described in British Standard Specification No. 8.82 were treated in the bath for 3 hours at 900 C.; on the 8th day no intergranular penetration of the surface layers was observed but intergranular penetration to a depth of 0.001 inch. was observed on the 16th and 21st days.

What we claim is:

1. A fused salt bath for the heat treatment of steels and alloy steels which comprises alkaline earth metal halide, alkali metal halide, 5% to 30% of alkali metal cyanide, and 0.25% to 5% of a regenerator selected from the group consisting of titanium and titanium dioxide, the percentages indicated being calculated on the total weight of the bath.

2. A fused salt bath as claimed in claim 1 in which the regenerator is in a form having a large ratio of surface to volume.

3. A fused salt bath as claimed in claim 1 containing 0.5% to 2% of titanium.

4. A fused salt bath as claimed in claim 1 containing 15% to 25% of barium chloride and 15% to 25% of sodium cyanide, the remainder being alkali metal chloride and regenerator.

5. A process for heat treating steel and alloy steel least one of said regenerators is used in combination which comprises heating the steel in a fused salt bath with silicon. comprising alkaline earth metal halide, alkali metal halide, 5% to 30% alkali metal cyanide, and 0.25% to References Cited in the of this Patent 5% of at least one regenerator selected from the group 5 UNITED STATES PATENTS consisting of titanium and titanium dioxide.

perature of the bath is in the range 800 C. to 950 C. gsg g et a1 6. A process as claimed in claim 5 in which the tem- 2364292 Holt 1944 perature of the bath is in the range 800 C. to 950 C.

7. A fused salt bath according to claim 1 wherein at 10 2711981 Waterfall et June 1955 

1. A FUSED SALT BATH FOR THE HEAT TREATMENT OF STEELS AND ALLOY STEELS WHICH COMPRISES ALKALINE EARTH METAL HALIDE, ALKALI METAL HALIDE, 5% TO 30% OF ALKALI METAL CYANIDE, AND 0.25% TO 5% OF A REGENERATOR SELECTED FROM THE GROUP CONSISTING OF TITANIUM AND TITANIUM DIOXIDE THE PERCENTAGE INDICATED BEING CALCULATED ON THE TOTAL WEIGHT OF THE BATH. 